Mold closure position detection method for mold clamping apparatus

ABSTRACT

In a mold closure position detection method, a mold closure position is detected on the basis of a variation in a physical quantity because of closure of a mold. The method comprises detecting an amount of movement of a movable platen, or a crosshead in the case where the mold clamping apparatus is of a toggle type, during closure of the mold; detecting a variation in the physical quantity because of closure of the mold; obtaining a rate of variation (which encompasses an amount of variation) in the physical quantity to a predetermined amount of movement of the movable platen or the crosshead; and detecting, as a mold closure position, a position of the movable platen or the crosshead when the variation rate reaches a preset ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mold closure position detectionmethod for a mold clamping apparatus, which method detects a moldclosure position on the basis of a variation in a physical quantitybecause of closure of a mold.

2. Description of the Related Art

A conventional toggle-type mold clamping apparatus for clamping a moldof an injection molding machine is disclosed in, for example, JapanesePatent Publication (kokoku) No. 6(1994)-61806. As disclosed in thispublication, a toggle-type mold clamping apparatus includes a togglelink mechanism which connects a movable platen for supporting a movablemold half and a crosshead advanced and retracted by a drive unit, andhas a function of transmitting pressing force of the crosshead to themovable platen while amplifying the force. In such a mold clampingapparatus, when the toggle link mechanism is almost completely extended,a predetermined mold clamping force determined on the basis of anextension of tie bars is generated. As shown in FIG. 10, in a moldclamping operation, high-speed mold closing is typically performed froma mold open position Xa, and the operation mode is switched tolow-speed, low-pressure mold closing at a predetermined low-speed,low-pressure changeover position Xb. The period during which thelow-speed, low-pressure mold closing is performed serves as a moldprotection zone, during which a molded product not having been properlyejected or the like is detected as a foreign object. When apredetermined high-pressure changeover position Xc is reached, theoperation mode is switched to high-pressure mold clamping so as to clampthe mold under high pressure. In FIG. 10, Xd shows a mold clamping endposition. Load torque T of a drive motor for driving the mold clampingapparatus changes as shown in FIG. 10 during the mold clampingoperation.

Incidentally, unlike a direct-pressure-application-type mold clampingapparatus, because of its operation principle, a toggle-type moldclamping apparatus has a drawback in that slight expansion orcontraction of a mold and tie bars, stemming from disturbing factorssuch as heating temperature of the mold and ambient temperature, causesa considerable variation in mold clamping force, which results indeterioration in quality, in particular at the time of molding ofprecision products. FIG. 11 shows a variation in mold clamping force Fmwith time for the case where the correct value (target value) of moldclamping force Fm is 400 kN. As is apparent from FIG. 11, during aperiod in which the mold temperature elevates, the mold clamping forceFm increases from 400 kN to 500 kN because of thermal expansion of themold. After completion of the temperature elevation, since heat istransferred from the mold to the tie bars, the tie bars expand, wherebythe mold clamping force Fm gradually decreases. Notably, thermalexpansion of the mold is a factor which increases the mold clampingforce Fm, and thermal expansion of the tie bars is a factor whichdecreases the mold clamping force Fm.

As described above, in a toggle-type mold clamping apparatus, disturbingfactors such as heating temperature of a mold and ambient temperatureare influential factors which must be taken into consideration so as toaccurately maintain the mold clamping force Fm. Japanese PatentApplication Laid-Open (kokai) No. 62(1987)-32020 discloses a moldclamping force control method for a toggle-type mold clamping apparatus,which method can cope with such disturbing factors. In the disclosedmethod, the thickness of a mold or a mold clamping force during amolding operation is detected by means of mold-thickness detection meansconsisting of an optical or magnetic scale supported on a stationarymold plate and a position detector disposed on a movable mold plate, anda correction value determined from the detected thickness and its targetvalue is fed back to mold-thickness adjustment means, whereby moldclamping force is maintained constant.

However, the conventional mold clamping force control method (moldclosure position detection method) for a toggle-type mold clampingapparatus has the following problems.

First, as described above, in a toggle-type mold clamping apparatus,slight expansion or contraction of a mold results in a considerablevariation in mold clamping force. Since the conventional mold clampingforce control method detects the thickness of a mold (mold clampingforce) by use of mold-thickness detection means consisting of a scalesupported on a stationary mold plate and a position detector disposed ona movable mold plate; i.e., the method detects slight expansion andcontraction, the method cannot accurately detect mold clamping force(mold closure position).

Second, since the thickness of a mold is detected directly, separatemold-thickness detection means such as a scale and a position detectorare needed, leading to an increase in the number of parts, higher cost,and increased degree of complexity of configuration; in particular, anincreased degree of complexity of the structure around a mold.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a mold closure positiondetection method for a mold clamping apparatus, which method can performmore accurate and consistent (stable) detection as compared with thecase where a physical quantity (absolute value) itself is compared witha threshold; more specifically, a mold closure position detection methodfor a mold clamping apparatus, which method enables accurate andconsistent detection without direct influence of disturbances such astemperature drift and mechanical friction, and which method greatlyreduces resetting and fine adjustment which are necessary when moldclosing speed or mold clamping force is changed upon replacement of amold.

Another object of the present invention is to provide a mold closureposition detection method for a mold clamping apparatus, which methodeliminates the necessity for mold-thickness detection means, such as ascale and a position detector, for directly detecting the thickness of amold at the time of detection of a mold closure position, to therebylower cost through reduction in the number of parts, and prevent thestructure around a mold from becoming complex.

Still another object of the present invention is to provide a moldclosure position detection method for a mold clamping apparatus, whichmethod enables accurate detection of a mold closure position whileeliminating influences of mold closing speed and other factors, tothereby enable accurate detection of variation in mold closing force andaccurate correction of mold clamping force.

To achieve the above object, the present invention provides a moldclosure position detection method for a mold clamping apparatus in whicha mold closure position is detected on the basis of a variation in aphysical quantity because of closure of a mold, the method comprising:detecting an amount of movement of a movable platen, or a crosshead inthe case where the mold clamping apparatus is of a toggle type, duringclosure of the mold; detecting a variation in the physical quantitybecause of closure of the mold; obtaining a rate of variation (whichencompasses an amount of variation) in the physical quantity to apredetermined amount of movement of the movable platen or the crosshead;and detecting, as a mold closure position, a position of the movableplaten or the crosshead when the variation rate reaches a preset ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing processing steps of the mold closureposition detection method according to an embodiment of the presentinvention, the steps being for a closure position detection mode to beperformed at the time of initial setting;

FIG. 2 is a flowchart showing the processing steps of a mold clampingforce correction method using the mold closure position detectionmethod;

FIG. 3 is a flowchart showing the processing steps for the closureposition detection mode which uses the mold closure position detectionmethod and which is performed at the time of production operation;

FIG. 4 is a flowchart showing processing steps for correcting moldingclamping force by use of the mold closure position detection method;

FIG. 5 is a view showing the structure of a toggle-type mold clampingapparatus for which the mold closure position detection method isperformed;

FIG. 6 is a block circuit diagram showing a portion of a molding machinecontroller provided for the toggle-type mold clamping apparatus, whichcontroller performs the mold closure position detection method;

FIG. 7 is a view showing a display screen of a display unit provided onan injection molding machine which performs the mold closure positiondetection method;

FIG. 8 is a graph showing variation in load torque with position of acrosshead, the graph being used for describing the mold closure positiondetection method;

FIG. 9 is a view showing an another mode of display of the display unitscreen provided on the injection molding machine which performs the moldclosure position detection method;

FIG. 10 is a graph showing variation in load torque with position of acrosshead, the graph being used for describing a conventional technique;and

FIG. 11 is a graph showing variation in mold clamping force with time,the graph being used for describing the conventional technique.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will next be described in detailwith reference to the drawings. The accompanying drawings areillustrative of the embodiment and are not meant to limit the scope ofthe invention. In order to describe the invention clearly, detaileddescription of known parts is omitted.

First, the structure of a toggle-type mold clamping apparatus Mc towhich a mold closure position detection method according to the presentembodiment can be applied will be described with reference to FIGS. 5 to9.

FIG. 5 shows an injection molding machine M including a toggle-type moldclamping apparatus Mc and an injection apparatus Mi. The toggle-typemold clamping apparatus Mc includes a stationary platen and apressure-receiving platen 12 which are separated from each other. Thestationary platen is fixedly mounted oh an unillustrated machine base,and the pressure-receiving platen 12 is mounted on the machine base insuch a manner that it can advance and retract. Four tie bars 13 extendbetween the stationary platen and the pressure-receiving platen 12.Front ends of the tie bars 13 are fixed to the stationary platen, andrear ends of the tie bars 13 pass through the pressure-receiving platen12. Adjustment nuts 15, which also serve as stoppers for thepressure-receiving platen 12, are in screw-engagement with male threads14 formed at the rear ends of the tie bars 13.

The adjustment nuts 15 constitute a mold-thickness adjustment mechanism16 for adjusting the position of the pressure-receiving platen 12. Thismold-thickness adjustment mechanism 16 further includes small gears 17coaxially and integrally provided on the respective adjustment nuts 15;a large gear 18 in meshing engagement with the small gears 17; a drivegear 19 in meshing engagement with the large gear 18; a mold-thicknessadjustment motor 20 having a rotary shaft on which the drive gear 19 isattached; and a rotary encoder 21 for detecting rotation of themold-thickness adjustment motor 20.

In this case, the small gears 17 are disposed at corresponding cornersof a square, and the large gear 18 is located to be surrounded by thesmall gears 17, so that all the small gears 17 are in meshing engagementwith the large gear 8 at all times. Therefore, when the mold-thicknessadjustment motor 20 is operated to rotate the drive gear 19, rotation ofthe drive gear 19 is transmitted to the large gear 18. Thus, the smallgears 17 rotate simultaneously, and the adjustment nuts 15, which rotatetogether with the corresponding small gears 17, advance or retract alongthe male threads 14 of the tie bars 13. As a result, thepressure-receiving platen 12 advances or retracts, whereby the positionof the pressure-receiving platen 12 in the forward/backward directioncan be adjusted.

Meanwhile, a movable platen 2 is mounted slidably on the tie bars 13.The movable platen 2 supports a movable mold half 1 m, and thestationary platen supports a stationary mold half 1 c. The movable moldhalf 1 m and the stationary mold half 1 c constitute a mold 1. A togglelink mechanism L is disposed between the pressure-receiving platen 12and the movable platen 2. The toggle link mechanism L includes a pair offirst links La pivoted on the pressure-receiving platen 12; a pair ofoutput links Lc pivoted on the movable platen 2; and a pair of secondlinks Lb pivotably coupled to connecting rods which connect the firstlinks La and the output links Lc. A crosshead 3 is coupled to the secondlinks Lb.

Moreover, a mold-clamping drive section 22 is disposed between thepressure-receiving platen 12 and the crosshead 3. The mold clampingdrive section 22 includes a ball screw mechanism 23, which consists of aball screw 24 rotatably supported on the pressure-receiving platen 12and a ball nut 25 in screw-engagement with the ball screw 24 and fixedto the crosshead 3; and a rotation drive mechanism section 26 forrotating the ball screw 24. The rotation drive mechanism section 26includes a servomotor 4 for mold clamping; a rotary encoder 5 attachedto the servomotor 4 so as to detect rotation of the servomotor 4; adrive gear 27 attached to a shaft of the servomotor 4; a driven gear 28attached to the ball screw 24; and a timing belt 29 wound around thedrive gear 27 and the driven gear 28.

By virtue of this configuration, when the servomotor 4 is operated, thedrive gear 27 rotates, and rotation of the drive gear 27 is transmittedto the driven gear 28 via the timing belt 29 so as to rotate the ballscrew 24, whereby the ball nut 25 advances or retracts. As a result, thecrosshead 3, with which the ball nut 25 is integrated, advances orretracts, and the toggle link mechanism L is contracted or expanded,whereby the movable platen 2 moves in a mold opening direction(retracting direction) or in a mold closing direction (advancingdirection). Reference numeral 30 denotes a molding machine controller,to which the mold clamping servomotor 4, the rotary encoder 5, themold-thickness adjustment motor 20, and the rotary encoder 21 areconnected.

FIG. 6 shows a servo circuit 31, which is a portion of the moldingmachine controller 30. The servo circuit 31 includes deviationcalculation sections 32 and 33; adders 34 and 35; a positional-loop-gainsetting section 36; a feed-forward-gain setting unit 37; a velocitylimiter 38, a velocity converter (differentiator) 39; avelocity-loop-gain setting section 40; a torque limiter 41; a driver 42;a disturbance monitoring section 43; and an acceleration converter(differentiator) 44. Thus, the system shown in FIG. 6 constitutes aservo control system (servo circuit 31). The above-mentioned moldclamping servomotor 4 is connected to the output side of the driver 42,and the rotary encoder 5 attached to the servomotor 4 is connected tothe inverted input sections of the velocity converter 39 and thedeviation calculation section 32. The non-inverted input section of thedeviation calculation section 32 is connected to an unillustratedsequence controller.

In FIG. 6, Pt denotes a signal output terminal used for detection ofload torque T generated at the time of closure of the mold 1; Pv denotesa signal output terminal used for detection of velocity V of the movableplaten 2 at the time of closure of the mold 1; Pa denotes a signaloutput terminal used for detection of acceleration A of the movableplaten 2 at the time of closure of the mold 1; Pe denotes a signaloutput terminal used for detection of estimated torque E generated bydisturbances at the time of closure of the mold 1; and Px denotes asignal output terminal used for detection of position deviation Xr ofthe movable platen 2 at the time of closure of the mold 1. Notably,operations (functions) of the respective sections will be described inthe following description of overall operation of the toggle-type moldclamping apparatus Mc.

FIG. 7 is a display screen 50 of a display unit attached to the sidepanel, or the like of the injection molding machine M. A touch panel isattached to the display screen 50, and various settings or otheroperations can be performed by use of this touch panel. The displayscreen 50 shown in FIG. 7 is a setting screen for mold opening andclosing, and includes a graphic display section 51 for graphicallydisplaying a curve W representing variation in load torque T. Thedisplay screen 50 further includes a numerical value display section 61in relation to a mold protection end position, a numerical value displaysection 62 in relation to a mold closure position, a numerical valuedisplay section 63 in relation to a mold closure reference position, anda numerical value display section 64 in relation to a mold closuremonitor end position.

In this case, the graphic display section 51 has a horizontal axis(X-axis) for position (mm) of the crosshead 3, and a vertical axis(Y-axis) for load torque T (%). Notably, the load torque T (%) isdisplayed such that the maximum torque is displayed as 100%. With thisconfiguration, the magnitude of the load torque T corresponding to theposition of the crosshead 3 is graphically displayed in the graphicdisplay section 51 in the form of a variation curve W. Further, in thegraphic display section 51, a cursor 61 c (pink) for indicating an endposition Xe of a mold protection zone Zd1, a cursor 62 c (red) forindicating a detection value Dd of a mold closure position Cs, a cursor63 c (blue) for indicating a reference value Ds of the mold closureposition Cs, and a cursor 64 c (green) for indicating an end position Xfof a mold closure monitor zone, which serves as a second mold protectionzone Zd2, are displayed by vertical lines of corresponding colors. Thecursors 61 c, 62 c, 63 c, and 64 c having different colors correspondingto the respective items enable an operator to easily and properly(reliably) know the positions corresponding to the respective items.

Meanwhile, the numerical value display section 61 includes a firstdisplay subsection 61 h for numerically displaying the end position Xeof the mold protection zone Xd1 (see FIG. 8) by a position of thecrosshead 3, and a second display subsection 61 x for numericallydisplaying a position of the movable platen 2 converted from thatposition of the crosshead 3. The numerical value display section 62includes a first display subsection 62 h for numerically displaying thedetection value Dd of the mold closure position Cs by a position of thecrosshead 3, and a second display subsection 62 x for numericallydisplaying a position of the movable platen 2 converted from thatposition of the crosshead 3. The numerical value display section 63includes a first display subsection 63 h for numerically displaying thereference value Ds of the mold closure position Cs by a position of thecrosshead 3, and a second display subsection 63 x for numericallydisplaying a position of the movable platen 2 converted from thatposition of the crosshead 3. The numerical value display section 64includes a first display subsection 64 h for numerically displaying theend position Xf of the mold closure monitor zone, which serves as thesecond mold protection zone Zd2, by a position of the crosshead 3, andan ON key 64 s for setting the end position Xf. The item names “moldprotection end position,” “mold closure position,” “mold closurereference position,” and “mold closure monitor” are displayed onrespective positions above the numerical value display sections 61, 62,63, and 64. Further, color frames 61 k (pink), 62 k (red), 63 k (blue),and 64 k (green) are displayed so as to surround the corresponding itemnames. The colors of these color frames 61 k (pink), 62 k (red), 63 k(blue), and 64 k (green) correspond to the colors of the cursors 61 c(pink), 62 c (red), 63 c (blue), and 64 c (green). By virtue of thiscolor arrangement, the operator easily discerns which items correspondto the cursors 61 c, 62 c, 63 c, and 64 c. Notably, position of thecrosshead 3 can be easily converted to position of the movable platen 2by use of a known conversion formula. Further, FIG. 9 shows a differentdisplay mode in which the scales of the horizontal axis (X-axis) andvertical axis (Y-axis) of the graphic display section 51 are changed.

Next, operation (function) of the toggle-type mold clamping apparatusMc, including a mold closure position detection method according to thepresent embodiment, will be described with reference to FIGS. 1 to 9.

The molding machine controller 30 has a closure position detection modefor detecting the mold closure position Cs. Notably, the mold closureposition Cs is a position at which the movable mold half 1 m and thestationary mold half 1 c come into mutual contact. In the closureposition detection mode, the molding machine controller 30 detects anamount of movement (displacement amount) of the crosshead 3 at the timeof closure of the mold 1, and a variation in a physical quantity at thetime of closure of the mold 1, obtains a rate of variation ΔT in thephysical quantity per the unit movement amount (predetermined movementamount) ΔX of the crosshead 3, and detects, as a mold closure positionCs, a position of the crosshead 3 when the variation rate ΔT reaches apreset threshold rate Ts.

In this case, the variation rate ΔT may be a variation amount. That is,the variation rate ΔT can be a variation amount ΔT per unit movementamount ΔX or a variation rate obtained from ΔT/ΔX. Further, load torqueT is used as a physical quantity. A signal indicative of the load torqueT is obtained from the signal output terminal Pt. The signal obtainedfrom the signal output terminal Pt is fed to the molding machinecontroller 30. Further, the movement amount of the crosshead 3 isdetected by use of encoder pulses output from the rotary encoder 5 fordetecting rotation of the servomotor 4.

Meanwhile, the above-mentioned threshold rate Ts is set in the moldingmachine controller 30. As shown in FIG. 9, the threshold rate Ts is usedto detect, as the mold closure position Cs, the position at which thevariation rate (increase rate) ΔT of the load torque T per the unitmovement amount ΔX of the crosshead 3 reaches the threshold rate Ts.Therefore, the threshold rate Ts can be properly set through anexperiment, adjustment, etc. Since the load torque T is displayed suchthat the maximum torque is displayed as 100% the threshold rate Ts canbe set as a percent value. For example, in the case where the unitmovement amount ΔT of the crosshead 3 is set to a few millimeters, andthe variation rate (increase rate) ΔT of the load torque T at that timeis obtained, the threshold rate Ts for the variation rate ΔT may be setto about 1%.

Next, specific processing steps will be described. First, detection ofthe mold closure position Cs is performed in the closure positiondetection mode. The processing steps for this closure position detectionmode will now be described in accordance with the flowchart shown inFIG. 1.

The mold 1 is assumed to be presently located at a mold open position(full open position). Therefore, the crosshead 3 of the toggle mechanismL is located at a mold open position Xa shown in FIG. 8. Upon start ofmold clamping operation, the mold clamping servomotor 4 is operated, sothat the crosshead 3 advances and the movable platen 2 advances from themold open position in a mold closing direction. At this time, high-speedmold closing, in which the movable platen 2 advances at high speed, isfirst performed.

In this case, the servo circuit 31 performs velocity control andposition control for the movable platen 2 (crosshead 3). That is, aposition instruction value is fed from the sequence controller to thedeviation calculation section 32 of the servo circuit 31, and iscompared with the position detection value obtained on the basis ofencoder pulses from the rotary encoder 5. As a result, a positiondeviation Xr is output from the deviation calculation section 32, andfeedback control for position is performed on the basis of the positiondeviation Xr.

The position deviation Xr is amplified by means of thepositional-loop-gain setting section 36 and fed to an input section ofthe adder 34. Moreover, the position instruction value is amplified bymeans of the feed-forward-gain setting section 37 and fed to anotherinput section of the adder 34. An output of the adder 34 is fed to anon-inverted input section of the deviation calculation section 33 viathe velocity limiter 38. Meanwhile, the position detection value isdifferentiated by means of the velocity converter 39 to thereby beconverted to a velocity (velocity detection value) V, which is fed to aninverted input section of the deviation calculation section 33. As aresult, a velocity deviation is output from the deviation calculationsection 33, and feedback control for velocity is performed on the basisof the velocity deviation. Notably, the velocity V is limited by meansof the velocity limiter 38.

The velocity deviation is amplified by means of the velocity-loop-gainsetting section 40 and fed to an input section of the adder 35.Meanwhile, the velocity V is differentiated by means of the accelerationconverter 44 to thereby be converted to an acceleration (accelerationdetection value) A, which is fed to an input section of the disturbancemonitoring section 43. The disturbance monitoring section 43 monitorsthe acceleration A. When the acceleration A anomalously changes becauseof a certain cause (disturbance), the disturbance monitoring section 43outputs an estimated torque (torque value) E for accelerating return tothe normal. This estimated torque E is fed to an input section of theadder 35 as a correction value. As a result, a torque instruction(instruction value) is output from the adder 35 and fed to the driver 42via the torque limiter 41. With this, the servomotor 4 is driven andcontrolled, whereby position control and velocity control for themovable platen 2 (crosshead 3) are performed. Notably, the torqueinstruction output from the torque limiter 41 is fed back to an inputsection of the disturbance monitoring section 43.

Meanwhile, the crosshead 3 reaches a preset low-speed, low pressurechangeover point Xb as a result of advancement of the movable platen 2in the mold closing direction, and operation for low-speed, low-pressuremold closing is started (step S1). The operation for low-speed,low-pressure mold closing is performed for mold protection (e.g.,foreign object detection) first in a mold protection zone Zd1, which isset before a second mold protection zone Zd2, which will be describedlater, as shown in FIG. 8. Specifically, in the mold protection zoneZd1, the magnitude of the load torque T is monitored. When the magnitudeof the load torque T exceeds a preset threshold, a foreign object isdetermined to be present, and processing for anomaly such as moldopening control is performed.

As shown in FIG. 9 (FIG. 7), the end position Xe of the mold protectionzone Zd1 is previously set by means of the numerical value displaysection 61, which has a setting function. Since this end position Xe isprovisionally set before detection of the correct mold closure positionCs (reference value Ds), the end position Xe can be set to be locatedbefore a predicted mold closure position with some margin. An operatornumerically sets, as the end position Xe, a corresponding position ofthe crosshead 3 by use of the first display subsection 61 h. In thiscase, a known setting method can be employed so as to allow the operatorto enter a numerical value by use of a ten-key window, which isdisplayed, for example, when the first display subsection 61 h istouched. In response to the entry of the end position Xe, the cursor 61c is displayed in the graphic display section 51 at a positioncorresponding to the end position Xe. As described above, the color(pink) of the cursor 61 c is the same as that of the color frame 61 kdisplayed in the numerical value display section 61. Therefore, theoperator can easily and correctly know the end position Xe from thecursor 61 c, and can easily and correctly know that the end position Xeis related to the mold protection zone Zd1.

When setting operation in relation to the mold protection zone Zd1 ends,processing for detecting the mold closure position Cs of the mold 1 isperformed. Specifically, increase rate monitoring process for monitoringthe increase rate ΔT is performed so as to detect the mold closureposition Cs (step SP). Notably, at this point in time the second moldprotection zone Zd1, shown in FIG. 8 and serving as a mold closuremonitor zone, has not yet been set. In the increase rate monitoringprocessing, the molding machine controller 30 first detects the positionof the crosshead 3 (step S2). The position of the crosshead 3 isdetected by use of encoder pulses output from the rotary encoder 5 fordetecting rotation of the mold clamping servomotor 4. In the presentembodiment, the rotary encoder 5 is of an incremental type, and detectsthe absolute position from the number of encoder pulses counted from areference position. Use of such a rotary encoder 5 eliminates necessityof separate position detection means for detecting the position of thecrosshead 3. As described above, the mold closure position Cs can bedetected accurately by making use of the displacement amount (movementamount) of the crosshead 3, whose movement amount is greater than thatof the movable platen 2, whereby accurate detection of the mold closureposition Cs becomes possible. As a result, the amount of variation inmold clamping force Fm, which will be described later, can be detectedaccurately, and thus, accurate correction of the mold clamping force Fmbecomes possible.

Further, the molding machine controller 30 acquires the load torque T atsampling intervals of, for example, 500 μsec, and obtains the average ofN sampled values of the load torque T through averaging processing(steps S3 and S4). As a result, as shown in FIGS. 7 and 9, the obtainedload torque T is graphically displayed in the graphic display section 51as a variation curve W, which shows variation in the load torque T withthe detected position of the crosshead 3.

Moreover, from the displacement amount (movement amount) of thecrosshead 3 and the variation amount of the load torque T, the increaseamount (increase rate) ΔT of the load torque T with respect to the unitmovement amount ΔX of the crosshead 3 is obtained (step S5). In thepresent embodiment, the unit movement amount ΔX of the crosshead 3 isset to a few millimeters, and a corresponding increase amount (increaserate) ΔT (%) of the load torque T is obtained. The increase rate ΔT ismonitored so as to determine whether the increase rate ΔT reaches thepreset threshold rate Ts. When the increase rate ΔT reaches the presetthreshold rate Ts, the position of the crosshead 3 at that time isacquired as the mold closure position Cs (steps S6 and S7). Further, theacquired mold closure position Cs is set or stored as a reference valueDs (step S8).

Meanwhile, the reference value Ds is displayed by means of the cursor 63c in the graphic display section 51 of the display screen 50. Asdescribed above, the color (blue) of the cursor 63 c is the same as thatof the color frame 63 k displayed in the numerical value display section63. Therefore, the operator can easily and correctly know the referencevalue Ds from the cursor 63 c, and can easily and correctly know thatthe reference value Ds is related to the mold closure position Cs. Thereference value Ds is numerically displayed in the first displaysubsection 63 h, and is converted to a position of the movable platen 2,which is displayed in the second display subsection 63 x. The above isthe basic operation of the closure position detection mode, and theactual reference value Ds (and detection value Dd) can be obtained byperforming the processing for the closure position detection mode aplurality of times and averaging a plurality of obtained closurepositions.

Further, while referring to the set reference value Ds, the operatormanually sets the end position Xf of the mold closure monitor-zone (thesecond mold protection zone Zd2) by use of the numerical value displaysection 64, which has a setting function (step S9). Specifically, theoperator numerically sets the end position Xf of the second moldprotection zone Zd2 in the display subsection 67 h, while referring tothe cursor 63 c which is displayed in the graphic display section 51 andis related to the reference value Ds, the numerical value displayed inthe first display subsection 63 h, etc. In this case, the end positionXf is set by the position of the crosshead 3. Further, the end positionXf is set in consideration of, in particular, the thickness of a moldedproduct. For example, in the case where the thickness of the moldedproduct is 0.1 mm, the end position Xf can be set to be located betweenthe mold closure position Cs and a position located before the moldclosure position Cs by 0.1 mm. In the present embodiment, the endposition Xf is set by the position of the crosshead 3, which moves in agreater amount than does the movable platen 2. Therefore, the endposition Xf can be set easily and accurately even in the case of asheet-like molded product having a thickness of about 0.1 mm.

After having set the end position Xf to the display subsection 64 h, theoperator touches the ON key 64 s, whereby the end position Xf of themold closure monitor zone, serving as the second mold protection zoneZd2, is displayed in the graphic display section 51 by means of thecursor 64 c. As described above, the color (green) of the cursor 64 c isthe same as that of the color frame 64 k displayed in the numericalvalue display section (mold closure monitor zone setting section) 64.Therefore, the operator can easily and correctly know the end positionXf from the cursor 64 c, and can easily and correctly know that the endposition Xf is related to the mold closure monitor zone, serving as thesecond mold protection zone Zd2.

Next, operation during production operation will be described inaccordance with the flowchart of FIG. 2 (FIG. 3 and FIG. 4).

Production operation is assumed to be presently performed in anautomatic molding mode (step S11). In this case, the above-describedreference value Ds has already been set. During production operation,when a preset time for detection of closure position or a preset numberof shots for detection of closure position is reached, operation for theclosure position detection mode is automatically performed (steps S12and S13). The intervals at which operation for the closure positiondetection mode is performed can be set in consideration of the degree ofchange in the mold clamping force F in an actual machine. For example,the operation for the closure position detection mode may be performedfor every shot or for every predetermined number of shots, or uponelapse of a predetermined period of time.

Next, the processing steps for the closure position detection modeduring the production operation will be described in accordance with theflowchart of FIG. 3. Now, the mold 1 is assumed to be presently locatedat a mold open position (full open position). Therefore, the crosshead 3of the toggle mechanism L is located at the mold open position Xa shownin FIG. 8. Upon start of mold clamping operation, the servomotor 4 isoperated, so that the movable platen 2 advances from the mold openposition in the mold closing direction. At this time, high-speed moldclosing, in which the movable platen 2 advances at high speed, is firstperformed (step S21). When the crosshead 3 reaches a preset low-speed,low-pressure changeover point Xb as a result of advancement of themovable platen 2 in the mold closing direction, low-speed, low-pressuremold closing is performed (step S22). In this low-speed, low-pressuremold closing, as shown in FIG. 8, processing for detecting anomaly suchas presence of a foreign object is performed in the mold protection zoneZd1.

Upon passage of the mold protection zone Zd1, in the mold closuremonitor zone serving as the second mold protection zone Zd2, monitoringprocessing for determining whether a molded product is caught betweenthe mold halves is performed (steps S23, S24, and S25). As describedabove, in the case where the thickness of a product to be molded is 0.1mm, the end position Xf of the mold closure monitor zone is set to belocated between the mold closure position Cs and a position locatedbefore the mold closure position Cs by 0.1 mm. This enables detection ofa thin sheet-like molded product remaining within the mold 1 withoutbeing ejected during mold opening. In actuality, even a sheet-likemolded product having a thickness of about 0.09 mm can be detectedwithout fail.

Meanwhile, after passage of the mold closure monitor zone, in theclosure position detection zone Zc, processing for detecting the moldclosure position Cs of the mold 1 is performed (steps S25 and SP). Thisdetection processing is the same as in step SP shown in the flowchart ofFIG. 1 and used for setting the reference value Ds. When the increaserate ΔT is detected to have reached the preset threshold ratio Ts by theprocessing for detecting the mold closure position Cs in the closureposition detection zone Zc, high-pressure mold clamping is performed soas to clamp the mold 1 under high pressure (step S26). Simultaneously,the position of the crosshead 3 when the increase rate ΔT reaches thepreset threshold ratio Ts is detected, and stores that position as adetection value Dd of the mold closure position Cs (steps S27 and S28).As can be understood from the above, setting of the mold protectionzones Zd1 and Zd2 and the closure position detection zone Zc enables theprocessing for detecting the mold closure position Cs to be performedafter passage of the mold protection zones Zd1 and Zd2. Therefore, theprocessing for protecting the mold 1 and the processing for detectingthe mold closure position Cs can be performed in a stable and reliablemanner while occurrence of interference therebetween is prevented.

The detected detection value Dd is numerically displayed in the firstdisplay subsection 62 h and the second display subsection 62 x of thenumerical value display section 62 of the display screen 50, and isdisplayed in the graphic display section 51 by means of the cursor 62 c.As described above, the color (red) of the cursor 62 c is the same asthat of the color frame 62 k displayed in the numerical value displaysection 62. Therefore, the operator can easily and correctly know thedetection value Dd from the cursor 62 c, and can easily and correctlyknow that the detection value Dd is related to the mold closure positionCs.

The operation for the closure position detection mode is performed apredetermined number of times, and the detection value Dd is obtainedfrom the average of a plurality of obtained values of the mold closureposition Cs (steps S13, S14, and S15). As a result, the obtaineddetection value Dd is highly reliable and free of noise. Subsequently, adeviation Ke of the detection value Dd from the preset reference valueDs; i.e. Ke=Ds−Dd, is obtained (step S16). After that, the end positionXe of the mold protection zone Zd1 and the end position Xf of the moldclosure monitor zone serving as the second mold protection zone Zd2 arecorrected on the basis of the deviation Ke. The correction of the endpositions Xe and Xf is performed as follows. Imaginary lines in FIG. 8show load torque variations Tf and Tr which occur when the mold clampingforce Fm changes. The load torque variation Tr represents a load torquevariation which occurs when the mold 1 is heated and thermally expanded;and a position before the correct mold closure position Cs is detectedas a mold closure position Cr. In this case, the mold clamping force Fmincreases. Accordingly, the end position Xf (Xe) is corrected such thatthe distance from the mold open position (origin) decreases by an amountcorresponding to the deviation Ke. Specifically, when the mold closureposition Cs shifts, the end position Xf (Xe) is corrected in such amanner that the above-described closure position detection zone Zcbetween the end position Xf (Xe) and the mold closure position Cs shownin FIG. 8 is maintained constant.

In particular, the load torque variation Tf shows the case where themovable mold half 1 m starts to come into contact with the stationarymold half 1 c before the end position Xf (Xe). In this case, ifcorrection of the end position Xf (Xe) is not performed, it becomesdifficult to determine whether the load torque variation Tr occursbecause of contact between the movable mold half 1 m and the stationarymold half 1 c during a normal operation or catching of a foreign objector the like, whereby erroneous detection may occur. However, when thecorrection of the end position Xf (Xe) is performed, the processing ofdetecting a foreign object or the like and the processing of detectingthe mold closure position Cs according to the present embodiment can beperformed in a stable and reliable manner, without occurrence ofinterference therebetween.

Similarly, the load torque variation Tf represents a load torquevariation which occurs when the tie bars 13 are heated and thermallyexpanded; and a position after the correct mold closure position Cs isdetected as a mold closure position Cf. In this case, the mold clampingforce Fm decreases. Accordingly, the end position Xf (Xe) is correctedsuch that the distance from the mold open position increases by anamount corresponding to the deviation Ke. In FIG. 8, Xd represents amold claming end position. Notably, even the mold closure positions Cs,Cf, and Cr, which relate to such variation in the mold clamping forceFm, can be accurately detected by means of the above-described closureposition detection mode.

Meanwhile, since an allowable range Re in relation to the deviation Kehas been previously set in the molding machine controller 30, theallowable range Re is compared with the deviation Ke so as to determinewhether the deviation Ke falls outside the allowable range Re. When thedeviation Ke falls within the allowable range Re, correction for themold clamping force Fm is not performed. Accordingly, the productionoperation is continued under the same conditions (steps S17 and S11).

When the deviation Ke falls outside the allowable range Re, thedetection value Dd is obtained again (steps S17, S18, and S13). That is,in the present embodiment, the detection value Dd is continuouslyobtained a plurality of times; and when the deviation Ke successivelyfalls outside the allowable range Re a plurality of times, correctionfor the mold clamping force Fm is performed (step S19 and S20). Forexample, correction for the mold clamping force Fm is performed when twodetection values Dd are successively detected and two deviations Keobtained therefrom fall outside the allowable range Re. Accordingly, inthe case where the deviation Ke falls outside the allowable range Reonly one time, the deviation Ke is determined to have been produced atemporary factor such as disturbance, and correction is not performed.This operation enhances the stability and reliability of correction.

Next, the processing steps for correction will be described inaccordance with the flowchart of FIG. 4. Since in the present embodimentcorrection is performed when the deviation Ke falls outside theallowable range Re a plurality of times (e.g., two times), a pluralityof the deviations Ke are obtained. Accordingly, in the presentembodiment, the deviations Ke are averaged so as to obtain a mean value(step S31). Notably, in the case where a plurality of deviations Ke aredetected, their mean value or the latest value may be used. Since thedeviation Ke is a deviation of the position of the crosshead 3, it isconverted to a deviation of the position of the movable platen 2 by useof a known conversion formula. With this operation, a correction amountKs for the movable platen 2 is obtained. The pressure-receiving platen12 is displaced by the correction amount Ks so as to perform correctionfor canceling the deviation Ke.

In this case, the correction processing is performed at a presetspecific timing without interruption of the molding cycle (step S32).Any of periods other than the high-pressure mold clamping period;specifically, the molding opening period, the ejection period, theintermediate period, or the like, can be used as the specific timingwhich does not interrupt the molding cycle. Accordingly, for example,the ejection period is assumed to be set as a period for performing thecorrection processing. A correction instruction is output at the startof the ejection period, and the correction processing is executed inaccordance with the correction instruction.

The correction processing is performed as follows. First, themold-thickness adjustment motor 20 is driven and controlled on the basisof the correction amount Ks, to thereby move the pressure-receivingplaten 12 in a direction for eliminating the deviation Ke (step S33). Atthis time, the pressure-receiving platen 12 is moved at a speed lowerthan the ordinary speed. The position of the pressure-receiving platen12 is detected by use of encoder pulses output from the rotary encoder21 attached to the mold-thickness adjustment motor 20, and feedbackcontrol for position is performed. The rotary encoder 21 is anincremental encoder; and the absolute position is detected on the basisof the number of generated encoder pulses counted from the referenceposition. When the pressure-receiving platen 12 has been moved to atarget position corresponding to the correction amount Ks (the deviationKe), the mold-thickness adjustment motor 20 is stopped (steps S34 andS35). By virtue of the above-mentioned automatic correction processing,timely and quick correction becomes possible. The correction processingmay be performed by making use of an existing automatic mold clampingforce setting function (automatic mold thickness adjustment function) ofthe toggle-type mold clamping apparatus Mc. The automatic mold clampingforce setting function is used, for example, at the time of moldexchange so as to set a target value of mold clamping force in aninitial stage, to thereby automatically set the mold clamping force.When such an existing automatic mold clamping force setting function isutilized, more accurate correction can be performed, and cost can belowered.

Notably, instead of the above-mentioned automatic correction, manualcorrection by an operator is possible. In this case, an allowable rangeRe is previously set for the deviation Ke, and when the deviation Ke hasdeviated from the allowable range Re, such even is reported by means ofwarning or the like. In response to this, the operator can performmanual correction. This enables the operator to perform correction bymaking use of his/her experience and know-how. In addition, depending onthe type of products to be molded, the operator can decide not toperform correction, while giving priority to production. Therefore,production operation (automatic molding) continues until the operatorperforms an operation for correction. Such correction modes can bepreviously selected by use of a selection key 71 on the display screen50 of the display unit.

In the mold closure position detection method for a clamping apparatusaccording to the present embodiment, since the mold closure position Csis detected by use of the variation rate ΔT, detection can be performedin a more accurate and more stable manner as compared with the method inwhich a physical quantity itself (absolute value) is compared with athreshold value for detection. That is, in the method in which aphysical quantity itself is compared with a threshold value fordetection, the detection is directly influenced by disturbances such asdrift, and therefore, accurate and stable detection is impossible. Inaddition, when mold closing speed or mold clamping force is changed as aresult of replacement of a mold, resetting and fine adjustment must beperformed frequently. Moreover, when resetting and fine adjustment areinsufficient, erroneous detection or detection failure occurs. Incontrast, in the present invention, since resetting and fine adjustmentare greatly reduced, such a drawback can be avoided. In addition, sincemold thickness detection means, such as a scale and a position detector,for directly detecting the thickness of the mold 1 becomes unnecessary,the number of parts can be reduced so as to lower cost. Moreover, thestructure around the mold 1 can be prevented from becoming complex.Moreover, the mold closure position Cs can be detected accurately bymaking use of the displacement amount (movement amount) of the crosshead3, whose movement amount is greater than that of the movable platen 2.As a result, the amount of variation in mold clamping force Fm can bedetected accurately, and thus, accurate correction of the mold clampingforce Fm becomes possible.

While the present invention has been described with reference to thepreferred embodiment, the present invention is not limited thereto.Regarding details of the method, configuration, numerical values, amongothers, modifications and any omission or addition may be possible asneeded without departing from the scope of the invention.

For example, anomaly such as catching of a foreign object encompassesnot only catching of a molded product between the mold halvesconstituting the mold 1, but also other different types of anomaliessuch as a failure or partial breakage of the mold 1 or the like. In theabove-described embodiment, the load torque T is detected by making useof the output (torque monitor) of the driver 42. However, the loadtorque T may be detected by making use of a torque instruction, which isan input of the torque limiter 41. When the closure position detectionmode processing and the correction processing are to be performed, ifnecessary, the automatic molding (production operation) may betemporarily stopped, and then resumed after completion of the closureposition detection mode processing and the correction processing. Ineach correction operation, correction may be performed by use of theentire correction amount Ks or a portion of the correction amount(Ks×k). Specifically, in the case where the control system becomesunstable (e.g., generation of hunting) as a result of correction by useof the entire correction amount Ks, correction may be performed by useof a correction amount which is obtained by multiplying the correctionamount Ks by a constant k smaller than 1 (in general, 0.1<k<1) and whichis made smaller than the deviation Ke; i.e., the value of Ks×k. In theabove-described embodiment, the load torque T, which changes uponclosure of the mold 1, is used as a physical quantity used for detectionof the mold closure position Cs. However, examples of other usablephysical quantities include velocity V of the crosshead 3 at the time ofclosure of the mold 1, acceleration A of the crosshead 3 at the time ofclosure of the mold 1, estimated torque E generated because ofdisturbance at the time of closure of the mold 1, and positionaldeviation Xr of the crosshead 3 at the time of closure of the mold 1.These physical quantities, including the load torque T, may be usedsingly or in combination. When these physical quantities are used incombination, reliability can be enhanced further. In the above-describedembodiment, the movement amount (displacement amount) of the crosshead 3is used as a displace amount of the movable platen 2. However, ifnecessary, the movement amount of the movable platen 2 may be directlyused.

1. A mold closure position detection method for a mold clampingapparatus in which a mold closure position is detected on the basis of avariation in a physical quantity because of closure of a mold, themethod comprising: detecting an amount of movement of a movable platen,or a crosshead in the case where the mold clamping apparatus is of atoggle type, during closure of the mold; detecting a variation in thephysical quantity because of closure of the mold; obtaining a rate ofvariation in the physical quantity to a predetermined amount of movementof the movable platen or the crosshead; and detecting, as a mold closureposition, a position of the movable platen or the crosshead when thevariation rate reaches a preset ratio.
 2. A mold closure positiondetection method for a mold clamping apparatus according to claim 1,wherein the moving amount of the movable platen or the crosshead isdetected by use of encoder pulses output from a rotary encoder fordetecting rotation of a mold clamping servomotor.
 3. A mold closureposition detection method for a mold clamping apparatus according toclaim 1, wherein the physical quantity is load torque at the time ofclosure of the mold.
 4. A mold closure position detection method for amold clamping apparatus according to claim 3, wherein variation in theload torque is graphically displayed in a graphic display section havinga horizontal axis for position of the crosshead.
 5. A mold closureposition detection method for a mold clamping apparatus according toclaim 4, wherein, in the graphic display section, a cursor indicatingthe detection value of the mold closure position and a cursor indicatingthe reference value of the mold closure position are displayed in thefrom of vertical lines of different colors.
 6. A mold closure positiondetection method for a mold clamping apparatus according to claim 1,wherein a position of the crosshead is displayed as the detection valueof the mold closure position by use of a numerical value displaysection, and a position of the movable platen converted from theposition of the crosshead is displayed by use of the numerical valuedisplay section.
 7. A mold closure position detection method for a moldclamping apparatus according to claim 1, wherein a position of thecrosshead is displayed as the reference value of the mold closureposition by use of a numerical value display section, and a position ofthe movable platen converted from the position of the crosshead isdisplayed by use of the numerical value display section.
 8. A moldclosure position detection method for a mold clamping apparatusaccording to claim 1, wherein a mold protection zone is set when themold is closed, and detection of the mold closure position is performedafter passage of the mold protection zone.